Tier 1 Internet Service Provider (Tier 1 ISP)

A tier 1 Internet service provider (Tier 1 ISP) is a type of ISP that directly connects with and has access to the global Internet backbone in a specific region under the settlement-free peering agreement, where the flow of information between one or more networks is exchanged voluntarily.

Techopedia explains Tier 1 Internet Service Provider (Tier 1 ISP)

Considered the highest ISP class, a tier 1 ISP has its own IP network in a particular region connected with the primary Internet backbone and/or other tier 1 ISP of same or different regions. It maintains the entire routing table for the Internet in its region.

Typically, a tier 1 ISP sells bandwidth to tier 2 and tier 3 ISPs, which, in turn, provide Internet connectivity to businesses and individual customers. Moreover, a tier 1 ISP binds an agreement with another tier 1 ISP for the free exchange of traffic and information. Thus, an ISP cannot be classified as tier 1 if it is required to pay a transit or peering fee to connect to a tier 1 ISP in the same global region.